1. Field of the Invention
The present invention relates to a toner and a developer using the toner for use in electrophotography.
2. Discussion of the Background
In an electrophotographic apparatus or an electrostatic recording apparatus, an electric latent image or a magnetic latent image is visualized with a toner. For example, in electrophotography, an electrostatic latent image formed on a photoreceptor is developed with a toner to form a toner image. The toner image is typically transferred onto a transfer material, and then fixed upon application of heat thereto. Typically, a toner for use in an electrostatic latent image development is a colored particulate material in which a colorant, a charge controlling agent, and other additives are dispersed in a binder resin. Toner manufacturing methods are broadly classified into pulverization methods and polymerization methods.
In a pulverization method, a colorant, a charge controlling agent, an offset-inhibitor, etc. are mixed and melt-kneaded with a thermoplastic resin, and then the mixture is pulverized and classified to prepare toner particles. Pulverized toners typically have properties on a reasonable level, however, materials that can be used for the pulverized toners are limited. For example, the melt-kneaded mixture has to be pulverized and classified using an economically usable apparatus. Therefore, the melt-kneaded mixture has to be brittle. In this case, particles having various particle diameters tend to be produced, i.e., the resultant toner has a broad particle diameter distribution. In order to produce high definition and high gradation images, for example, fine particles having a particle diameter of not greater than 5 μm and coarse particles having a particle diameter of not less than 20 μm have to be removed, resulting in deterioration of the toner yield. In addition, it is difficult to uniformly disperse toner components (such as a colorant and a charge controlling agent) in a thermoplastic resin in the melt-kneading process. Further, the colorant tends to present at the surface of the toner, and therefore charge quantity distribution of the toner broadens, resulting in deterioration of developability. Pulverization toners have insufficient toner properties to be used for high-performance image forming apparatuses.
In attempting to solve the above-mentioned problems of the pulverization method, suspension polymerization methods have been proposed. It is known that spherical toner particles are obtained by a suspension polymerization method. However, spherical toners have poor cleanability. When an image having low image proportion is formed on an image bearing member and then transferred, toner particles hardly remain on the image bearing member. In contrast, when an image having high image proportion is formed on an image bearing member and then transferred, toner particles tend to remain on the image bearing member and thereby the produced images have background fouling. Such residual toner particles also contaminate a charging roller configured to charge a photoreceptor, resulting in deterioration of charging ability thereof.
The suspension polymerization method has another drawback so as to have low flexibility in choosing raw materials for use therein. Since the binder resin is limited to resins which can be formed by polymerization reaction at a time of forming toner particles, almost all the resins which are conventionally used for toners cannot be used for the suspension polymerization method. In addition, particle diameter distribution of the toner cannot be well controlled due to the existence of internal additives (such as colorants) in some cases. In particular, the largest problem is that polyester resins, which can impart good fixability and color reproducibility to the resultant toner, cannot be used for the suspension polymerization method.
In attempting to solve these problems, Japanese Patent No. 2537503 discloses a toner manufacturing method in which fine resin particles obtained by an emulsion polymerization are associated to form toner particles having irregular shapes. (This method is hereinafter referred to as emulsion aggregation method, and the resultant toner is hereinafter referred to as emulsion aggregation toner.) However, a large amount of surfactant remains both on the surface of the toner particles and inside of the toner particles even after the toner particles are subjected to a washing process. As a result, the resultant toner has poor environmental stability in chargeability and broad particle diameter distribution, and thereby background fouling tends to occur in produced images. In addition, the residual surfactant contaminates image forming members (such as photoreceptor, charging roller, and developing roller). Although colorant particles hardly present at the surface of the resultant toner, the colorant particles are easily aggregated in the toner. In other words, it is difficult to uniformly disperse colorant particles in the emulsion aggregation toner. As a result, the resultant toner has an uneven chargeability, resulting in deterioration of charging stability after long repeated use. If developability and transferability of color toners slightly deteriorate, color balance and gradation of the resultant color images also deteriorate. When colorant particles are aggregated, light is diffusely reflected at the surface of the aggregated colorant particles, resulting in deterioration of transparency of toner images. When such toner images are formed on overhead projection (OHP) sheet, the projected images have poor color reproducibility.
Full-color image forming apparatuses typically use toners including a release agent without using an oil supplying device which applies an oil to the fixers. However, it is difficult to prepare a release agent having as small a particle diameter as colorants, and it is more difficult to uniformly disperse such a small release agent in the toner. When the release agent is not uniformly dispersed, chargeability, developability, and preservability of the toner, and transparency of toner images deteriorate.
Conventional yellow toners typically include dichlorobenzidine pigments (such as C. I. Pigment Yellow 17) as colorants. However, since the use of dichlorobenzidine is restricted in Germany and a product including dichlorobenzidine cannot obtain Blue Angel Mark, which is an ecology mark in Germany, a need exist for toners including no dichlorobenzidine pigment. Specific examples of yellow colorants including no dichlorobenzidine include C. I. Pigment Yellow 155, C. I. Pigment Yellow 180, C. I. Pigment Yellow 93, C. I. Pigment Yellow 74, etc. Some of these pigments show too high a structural viscosity when the pigment is dispersed in a solvent. In this case, it is difficult to obtain toner particles by the above-mentioned polymerization methods. On the other hand, some of these pigments have compatibility with water. In this case, the pigments cannot be held in toner particles and move into the water.
Polymerization methods except the emulsion aggregation methods typically produce spherical toner particles. Since spherical toner particles have small adhesion to a photoreceptor and easily release therefrom, the spherical toner particles can be sufficiently transferred. Moreover, adhesion among the spherical toner particles is small, and therefore each of the spherical toner particles is easily influenced by electric force. Therefore the toner particles adhere a latent image along the electric flux line, and thereby a toner image faithful to the latent image can be produced. However, such spherical toner particles tend to roll on a transfer paper when the toner particles contact a fixing member, resulting in producing abnormal images. In addition, since the spherical toner particles tend to roll on a photoreceptor, it is difficult to remove the spherical toner particles remaining on the photoreceptor using a cleaning blade.
Published unexamined Japanese Patent Applications Nos. (hereinafter referred to as JP-A) 09-179331, 10-142835, and 11-327197 have disclosed toners having specific shape factors SF-1 and/or SF-2. It is described therein that by controlling the shape factors, a good combination of toner properties such as chargeability, developability, transferability, and cleanability can be imparted to the resultant toner.
JP-A 2001-51444 discloses a toner having a specific shape factor and a specific surface are a ratio defined by the following equation: R=ρ×D50p×S, wherein R represents a surface area ratio, ρ (g/m3) represents the specific gravity of the toner, D50p (m) represents the number average particle diameter of the toner, and S (g/m2) represents the BET specific surface area of the toner. The surface area ratio R represents irregularity (i.e., degree of concavity and convexity) of the surface of the toner, which is an evaluation measure different from the above shape factors. It is described in JP-A 2001-51444 that a toner having too large a surface area ratio R has too large irregularity and such a toner causes a problem in that external additives are embedded in concavities and therefore the toner cannot maintain good chargeability and transfer ability for along period of time.
Because of these reasons, a need exists for a yellow toner which can be used for high-performance image forming apparatuses.